Using a laybarge to lay pipeline in a marine environment is a common occurrence. Further use of pipeline stingers to guide and support the pipeline before it is possitioned on or in the seabed is also of great assistance in this venture. During the laying operation, the pipeline becomes axially tensioned for several reasons. The primary reason is the need to control the bending stresses in the overbend and sagblend pipeline segments relative to the stinger.
Since the pipeline is continuously tensioned along the entire installation route, it retains all or a portion of this axial tension after being installed. Such `residual axial tension` is maintained in the pipeline because of the distributed friction forces between the pipeline and the adjacent seabed. Ordinarily such residual axial tension is of little consequence, however, should the pipeline be laid over an undulating seabed, the pipeline has a tendency to span over such undulations.
When the span length is small relative to pipeline diameter, the concern is minimal, but when the span length approaches or exceeds the regulatory standard of 75 to 150 feet, corrections and/or adjustments must be made. In fact, should these regulatory limits be exceeded, the unsupported pipeline span must be reduced otherwise there is risk of rupture or other physical damage to the pipeline.
The current practice, when an excessive length span occurs, is to use divers to erect underwater supports for the pipeline. One method of accomplishing this requires the diver to regularly insert grout bags underneath the laid pipe to provide support. Another method entails the use of structural supports that the diver must install along the unsupported span. These practices are feasible provided the diver can work for long periods at the necessary depth. Also, the remedial use of these supports becomes economically prohibitive should the span be lengthy or should the pipeline be more than 3 to 4 feet above the seabed.
Another alternative for providing pipeline support is the utilization of remotely operated vehicles (ROV's) to construct the needed supports. Such ROV span corrections, however, will be limited by the capabilities of the ROV to erect the pipeline support beneath the pipeline.
It is thus an object of this invention to provide a method of laying pipeline in a marine environment in such a manner that local `residual axial tension` is either eliminated or severely reduced. Another object of this invention is to provide a method that is considerably easier to deploy and much more economical than any of the methods now being utilized. A further object of this invention is to provide post-installation construction procedures that reduce or eliminate any excessive pipeline spanning that would otherwise occur over an undulating seabed. Yet another object of this invention is to enable an underwater pipeline to conform to the variable contours of the seabed with little or no spanning. These and other objects will become obvious upon further investigation of this invention.